The present invention relates to nuclear power plants, and in particular, to on-line monitoring of control rod positions relative to regulatory requirements for short term, long-term, and transient insertion limits.
Commercial nuclear power plants are subject to comprehensive regulatory compliance covering virtually every phase of nuclear reactor operation. Many of these regulatory constraints are manifested in the form of so-called xe2x80x9cTechnical Specificationsxe2x80x9d, which are an integral part of the operating license for the power plant. Each vendor of a nuclear steam supply system (NSSS), achieves compliance with the technical specifications, by formulating and justifying operating procedures for approval by the regulatory authorities.
In pressurized water nuclear power plants (PWR plants), one type of Technical Specification concerns the accumulated time during which control rods are present in the reactor core. As is well known, control rods serve two important functions. The extent of insertion directly affects the gross power level in the reactor core. Another function is to control the local distribution of power in the core, thereby avoiding high localized power peaking, relative to the average power generated in the core. The prolonged insertion of particular control rods in the core, especially during periods of relatively high power, can have two detrimental effects long term. First, the pattern of fuel consumption can be distorted to the extent that upon removal of these rods, new, previously unpredicted local power peaking or power oscillations may arise. Furthermore, control rods can prematurely lose effectiveness over time.
It is also well known that individual control rods can be ganged together as an assembly for insertion and removal by a single drive mechanism, and that a plurality of assemblies, such as four or eight, can be controlled as a group for substantially simultaneous movement into and out of the core. Four or five groups are typically programmed for staggered insertion and withdrawal from the core (unless, of course, all groups are to be dropped simultaneously to trip, or xe2x80x9cscramxe2x80x9d, the reactor). For purposes of the present disclosure, a cluster of control rods which are moved by a single drive mechanism, are referred to as a xe2x80x9ccontrol element assemblyxe2x80x9d (CEA), whereas a plurality of CEA""s which are controlled for substantially simultaneous movement into and out of the reactor, are referred to as a xe2x80x9cCEA groupxe2x80x9d.
According to one approach for compliance with Technical Specifications, plant Limiting Conditions of Operation (LCO) are established to impose operational constraints with regard to CEA rod group insertions and thereby assure that the design bases which underlie the Technical Specifications are not violated. These limitations are typically characterized in terms of restrictions imposed on CEA rod group insertions between the Long Term Steady State Insertion Limit (LTSSIL) and the Transient Insertion Limit (TIL). These restrictions are typically expressed in terms of either clock hours, or effective full power days (EFPD) of exposure. An EFPD is the exposure equivalent of 24 hours at the licensed full power operation of the reactor. In addition, restrictions are imposed upon exceeding the Short Term Steady State Insertion Limit (STSSIL) under certain conditions. In a PWR, all CEA""s are typically out of (above) the core at full steady state power, and are inserted downwardly into the core to reduce power level. Typical examples of limiting conditions of operation are set forth in the following Table 1.
These restrictions limit the duration (in terms of hours) that CEA rods can be positioned between the STSSIL and the TIL, and the amount of CEA exposure which can be accumulated (in terms of Effective Full Power Hours) for insertions between the LTSSIL and the TIL. The graph of FIG. 20 depicts typical operational regions bounded by these insertion limits.
The LTSSIL is a position limit in which there is no restriction for CEA rod insertions which are above this position. However, CEA rod insertions below this position and bounded by the TIL are constrained to the limits of CEA exposure as noted in Table 1.
The STSSIL is a position limit below (i.e., greater than) the LTSSIL in which further restrictions on insertion (time durationxe2x80x94as opposed to CEA exposure accumulations) are imposed on CEA rod insertions which are below this position and bounded by the TIL. These limits are noted in Table 1.
The TIL is a position limit below the STSSIL which CEA rod insertions must not exceed. This limit is designed to allow for plant maneuvering using CEA insertions (as long as the CEA""s do not go below this limit and as long as they maintain the CEA exposure and time limit durations for insertion as previously noted). Should CEA""s be inserted below the TIL, the plant annunciator system normally outputs an alarm message and the operator must then take corrective action (such asxe2x80x94restore the CEA rods to within the prescribed limits within a defined time period; or reduce plant thermal power).
It is conventional to identify groups of CEA""s beginning with number 1 and proceeding, e.g., to number 5 according to the order in which they are withdrawn from the core in a zero power condition at which all CEA groups are fully inserted. The corollary is that in the initial condition where the reactor core is at full power, with all rods out (the most desirable operating condition), Group 5 is the first to be inserted, followed by 4, 3, etc.
The Long-Term Steady State Insertion Limit is shown in FIG. 20 as a vertical line extending through range of 1.0-0.2 power fraction and (when projected) intersecting the Group 5 insertion representation bar at an insertion distance of 108 inch (274 cm), out of a total group rod length of 150 inches (381 cm). Because Group 4 and subsequent groups follow in staggered relationship, it is clear that whenever Group is positioned in the core anywhere within the Steady State Insertion Limit, no other Groups are in the core. It is evident that Group 4 does not begin entering the core, until Group 5 is at the 60 inch (152 cm) withdrawal position (i.e., 90 inches (229 cm) of insertion).
The Short Term Steady State Insertion Limit for Group 5 is also shown in FIG. 20 as a vertical line which has an upper limit at a power fraction of 0.75 and extends downward to 0.25, and intersects the Group 5 bar at the 60 inch (152 cm) withdrawal position. Thus, it can be appreciated from FIG. 20, that the Group 5 Short Term Steady State Insertion Limit, would not be accompanied by a Short Term Steady State Insertion Limit for any other Group.
On the other hand, the Transient Insertion Limit allows for a variety of CEA insertion configurations including the fifth and fourth Groups fully inserted and the third Group inserted at the 60 inch (152 cm) withdrawal position. Not all configurations are permitted at every power level, however, i.e., the greater extent of Group insertion, the lower the permitted power level even during a transient.
Thus, it may be appreciated that the LCO""s impose concurrent limitations on insertion. For example, even if the CEA groups have not reached the limit of 5 EFPD per 30 EFPD interval, for insertion between the LTSSIL and the TIL, desirable repositioning of the Groups may be foreclosed by the further requirement that insertion between the STSSIL and the TIL must not exceed 4 hours per 24 hour interval.
The foregoing operational requirements are presently maintained by manual surveillance. The inventor has concluded that this approach has the following shortfalls which are remedied by the present invention:
1. Manual monitoring is cumbersome and prone to human error.
2. There is no automatic method to display and analyze the monitored data which, in turn, reduces the situational awareness for the operator of the existing accumulated CEA group exposures relative to the operational limits.
3. There is no automatic early notification of approach to operational limits so that corrective action can be taken prior to exceeding an operational limit.
4. There is no automatic alarm notification when the operational limits are exceeded so that corrective action may be immediately initiated.
5. The resolution of the manually recorded data is coarse.
6. Manual recording of accumulated EFPD and hours for CEA rod group exposures does not conveniently lend itself to monitoring a contiguous data interval or window. This may result in the selection of discrete monitoring intervals which are sequential. Such discrete monitoring intervals can lead to potential circumscribing of the intent of the operational limits. For example, the restriction of 5 EFPD per 30 EFPD will seemingly be satisfied by two sequential monitoring intervals in which 4.5 EFPD exposure occurs during the last 4.5 days of the first monitoring interval (of 30 EFPD) and in which 4.5 EFPD exposure occurs during the first 4.5 days of the following monitoring interval (of 30 EFPD). Each monitoring interval seemingly satisfies the restriction of 5 EFPD per 30 EFPD interval but, in fact, 9 contiguous EFPD of exposure have occurred. If the starting period of the first monitoring interval was advanced 5 EFPD, then the total EFPD exposure for the first monitoring interval would have been 9 EFPD (rather than 4.5 EFPD) which exceeds the operational limit.
In this example, the operational limits were either complied with or violated depending upon the happenstance of when the start of a discrete monitoring interval was chosen.
According to the present invention, these deficiencies in conventional techniques are overcome by a method and apparatus, in which the incremental effective exposure for each CEA group is computed commensurate with core power, for each time increment at which each group is within the position range where an administrative limit is imposed. The increments of effective exposures for each group are accumulated, and the accumulated effective exposure for each group is compared with the administrative limit for each group. This comparison is then displayed to the reactor operator.
The displaying of the comparison to the reactor operator, preferably provides for continuous monitoring, alarming, and reporting of accumulated group exposure, expressed in terms of hours and effective full power days relative to the established operational limits. Although the administrative limits are preferably LCO""s, other administrative limits, whether or not based directly on the plant Technical Specifications, can provide the applicable limits.
In a further preferred embodiment, the display provides graphical information utilizing a xe2x80x9crolling wheelxe2x80x9d and xe2x80x9csliding barxe2x80x9d format. In a still further preferred embodiment, a display sectoring mode is included.
In yet further embodiments, query and predictive modes of operation, pre-alarm notification upon approach to applicable limits, and alarm notification upon exceeding applicable limits, are also provided. In the predictive mode, the effect on the LCO""s of a planned power maneuver is assessed. If insufficient EFPD margin is available, a projection is made as to when suitable margin will be regained to allow the maneuver to occur while maintaining compliance with the LCO""s.
In the pre-alarm feature, an early warning of an indication of approach to an LCO limit regarding accumulated EFPD is displayed, so that action can be taken to avoid an actual violation of the LCO.
The invention is preferably implemented to receive as continuous inputs: the current plant power level; the CEA Group positions; and the operational status of the Core Operating Limit Supervisory System (COLSS). The COLSS determine automatically and on-line, the gross thermal power level of the core. One implementation of such as system is described in U.S. Pat. Nos. 3,752,735, issued Aug. 19, 1973, and 4,330,367 issued May 18, 1982, the disclosures of which are hereby incorporated by reference. An internal clock maintains an accurate time base so that plant EFPD may be calculated as a function of the current plant power level, accumulated time, and licensed full power level. Accumulated time (in terms of hours), is also maintained employing the internal clock.
Utilizing the positions of the Regulating and Part Strength rod groups, and the internally calculated EFPD and accumulated time, the system continuously determines the exposure for these groups whenever they are inserted between the Long Term Steady State Insertion Limit and the Transient Insertion Limit. The exposures are determined for contiguous monitoring intervals which are defined by the Limiting Conditions for Operations (see examples in Table 1). The computed exposures are then continuously compared with the operational criteria.
In addition, the positions of the Regulating groups are continuously compared with the Short Term Steady State Insertion Limit whenever the applicable LCO""s are exceeded (such as whenever the Core Operating Limit Supervisory System is out of service). For such occurrences, excursions beyond the Short Term Steady State Insertion Limit are annunciated and the time remaining to take corrective action, in accordance to the Technical Specifications for operations, is displayed.
For cases in which the Limiting Conditions for Operation are not applicable (such as for a Reactor Power Cutback event) an inhibit signal prevents unwanted exposure accumulations or spurious alarm messaging. A reactor power cutback system of the type mentioned herein, is described in U.S. Pat. No. 4,075,059 issued Feb. 21, 1978, the disclosure of which is hereby incorporated by reference. It should be appreciated that the use of part strength CEA""s is an option, and the implementation of the invention follows the same procedures for part strength CEA""s as for regulating CEA""s. As the term is used herein, regulating CEA""s is meant to include all the Groups which are normally controlled for sequential insertion and removal, as depicted in FIG. 20, for the purpose of regulating power and/or power distribution during power generation in the reactor. The reactor may also have additional control rods which are not normally intended for regulating purposes, but which are available for rapid shutdown or extended zero power outages.
These features of the invention provides significant advantages over conventional techniques.
Automatic calculation and continuous display of accumulated time (hours) and accumulated Effective Full Power Days (EFPD) of CEA rod group exposure relative to the Limiting Conditions for Operation (LCO) for insertion between the Long Term Steady State Insertion Limit and the Transient Insertion Limit, is provided. Real-time monitoring of plant power and CEA rod group positions allows automatic and continuous calculation and updating of Effective Full Power Days and rod group exposures. This simplifies the operator workload and provides timely information relative to monitoring compliance with operational limitations on CEA rod group insertions and assists with the planning of future CEA rod group maneuvers.
Continuous comparison of Regulating rod group positions with the Short Term Steady State Insertion Limit under applicable conditions as noted within the LCO""s (such as whenever COLSS is out of service), provides automatic notification of exceeding the Limiting Condition for Operations for Regulating rod groups.
Graphical representation of accumulated time (hours) and Effective Full Power Days (EFPD) relative to the LCO""s, utilizing unique xe2x80x9cRolling Wheelxe2x80x9d and xe2x80x9cSliding Barxe2x80x9d display formats, is intuitive. The display formats provide the user with an easily understood representation of the accumulated time and accumulated EFPD exposure for CEA rod groups relative to the operational limits as defined by the LCO""s. The display formats are designed to accommodate a contiguous monitoring interval in which old exposure data is continuously discarded (rolls off for the xe2x80x9cRolling Wheelxe2x80x9d format or slides off for the xe2x80x9cSliding Barxe2x80x9d format) while new data is continuously added (rolls on for the xe2x80x9cRolling Wheelxe2x80x9d format or slides on for the xe2x80x9cSliding Barxe2x80x9d format) for the monitoring intervals as defined by the LCO""s. These graphical displays provide a spatial representation of accumulated rod group exposure for a contiguous monitoring interval which is readily understandable to the end user. The displays improve the situational awareness and comprehension of the existing accumulated rod group exposures and readily indicates when exposure margin can be regained.
The Sector mode which is associated with the graphical displays allows users to define xe2x80x9csectorsxe2x80x9d within the xe2x80x9cRolling Wheelxe2x80x9d and xe2x80x9cSliding Barxe2x80x9d displays to be expanded and thus examined at higher resolutions. The ability to xe2x80x9csectorxe2x80x9d to finer resolutions allows finer detail to be observed, for the interval of interest, than can normally be displayed on a Video Display Unit.
The Predictor Mode of operation allows the effect of a planned CEA Rod Group maneuver on the LCO""s to be assessed in advance of performing the actual maneuver. This minimizes the likelihood of exceeding the operational limits for CEA rod group exposure. If insufficient time (hours) or EFPD margin is available, the Predictor Mode projects when suitable margin will be regained to allow the maneuver to occur while maintaining compliance with the LCO""s. Various xe2x80x9cwhat ifxe2x80x9d scenarios can be investigated using the Predictor Mode.
The Query Mode allows the user to recall historic information and to determine when a certain level of accumulated exposure (in terms of hours and/or EFPD) will xe2x80x9croll off (slide off)xe2x80x9d and be regained as usable margin. This allows the user to review previously recorded information and to determine when accumulated exposure margin (expressed in terms of hours and/or EFPD) will be regained which serves as an advanced planning tool.
In the event of a system outage, the Update mode allows the system to be recalibrated to the current operational conditions. In event of a system outage the Update mode allows the user to enter the appropriate timexe2x80x94powerxe2x80x94and rod group exposure history for the outage interval in order to recalibrate the system to the current operational conditions. Thus, the system can account for outages and be immediately reinserted into service when the system is restored to operation.
The Summary Report allows the user to observe the accumulated exposure and remaining exposure margin for all Regulating and part Strength rod groups on a single display. Provides the user with an overall assessment of the current accumulated exposure and remaining exposure margin utilizing a single convenient display page. This alleviates the necessity of searching through multiple display pages to obtain an overall assessment of the current operational status.
The pre-alarm notification alerts the user to an impending approach to an established Limiting Condition for Operation. Advanced notification of an impending limit excursion provides the user with time to take corrective action before the limit is actually exceeded.
Alarm notification alters the user to any excursion beyond an established LCO boundary. Such alarming alerts the operator that an operational limit has been exceeded to that he may take appropriate action as called forth within the Technical Specifications for plant operations.
Time remaining for completion of corrective action is displayed whenever an Alarm is annunciated (via exceeding an LCO boundary). Display of such information provides the operator with a convenient assessment of the progress of corrective action(s) relative to requisite xe2x80x9cCompletion Timesxe2x80x9d as stated within the Technical Specifications for plant operation.
In cases for which the Limiting Conditions for Operation are not applicable (such as for a Reactor Power Cutback event) an inhibit signal prevents unwanted exposure accumulations or spurious alarm messaging.
A contiguous monitoring interval is maintained for calculating rod group exposure relative to the LCO""s for insertions between the Long Term Steady State Insertion Limit and the Transient Insertion Limit, rather than sequential discrete intervals. A contiguous monitoring interval avoids potential ambiguity in determining compliance with the LCO""s.